Certain industrial processes depend on well controlled flows of gas. An example is in the field of semiconductor device manufacturing, which uses a wide variety of gases for processing silicon wafers into integrated circuits. Plasma etching is a particularly important semiconductor process that depends upon use of flows of a number of different gases. In plasma etching processes, various gases are introduced into a vacuum chamber and electrical power (typically in the form of radio frequency excitation) is used to ignite a plasma that creates reactive gas species. These reactive gas species etch patterns into the silicon wafer to define different components of the integrated circuit.
Because of the extremely small dimensions of the components of modern integrated circuits, effective manufacturing requires the use of gas flows exhibiting very stable and consistent characteristics, for example mass flow as measured in standard cubic centimeters per minute (sccm). Typically however, the electro-mechanical mass flow controllers used to control the flow of gases are prone to drift over time. The semiconductor industry is especially sensitive to these drifts since variations as small as a few percent can severely degrade the performance of the integrated circuit.
Accordingly, maintenance of stable gas flows requires frequent calibration of the mass flow controllers. Conventionally, calibration of the mass flow controllers is accomplished by introducing the gas into a vacuum chamber of a known volume while monitoring the pressure within that chamber. Based upon the known correlation between pressure, volume, and the mass of the gas introduced (which defines the number of molecules of the gas), the rise in pressure as the gas flows into the vacuum chamber can be monitored. This information regarding pressure change within the chamber can then be used to calibrate the mass flow controller.
One potential disadvantage of this conventional approach to calibrating gas flows is a loss in throughput. Specifically, the gas flow calibration procedure consumes highly valuable equipment time during which no productive processing can take place.
From the above, it is seen that improved gas flow calibration techniques are desired.